Current digital technologies are widely used in the production, transmission, storage and playback of images and video. Digital processing of images and video offers numerous advantages over analog video, including improved quality, efficient transmission using compression, a variety of storage media, and the convenient organization of content. As a result, images and video are now largely distributed digitally using mediums such as digital versatile discs (DVDs). In addition to DVDs, higher resolution formats such as high definition DVD (HD-DVD) and Blu-ray have become increasingly popular formats for movie distribution over the last few years.
In networked environments such as the Internet or local area networks, digital content can be easily downloaded to a client device (for example, a client computer's hard disk) from content servers. The trend toward digital distribution of multimedia content has thus been helped by the explosive growth of the Internet as a medium of communication over the last number of years. The ability to generate and store digital content inexpensively has in turn helped expand the reach of the Internet.
Video and image data are often compressed prior to being written onto storage media such as hard disks, flash memory, and DVD to reduce storage requirements; or prior to transmission to save transmission bandwidth. At a receiver, encoded video or image data is decoded and sent to a display device. Typical decoders include DVD players, HD-DVD players, Blu-ray players, portable digital video players, personal computers equipped with video player software and the like.
Part of the reason for the increasingly widespread adoption of digital transmission and storage of video is the ability to use error control codes such as forward error correction (FEC) codes, cyclic redundancy checks (CRC) and the like, to detect and sometimes correct corrupted data. Received data may be corrupted as a result of transmission errors or due to storage media defects.
Error control coding involves the controlled introduction of redundancy in the transmitted (or stored) data stream at a transmitter, in such a manner that allows a receiver to detect and sometimes correct erroneously received data. However, the use of error correcting codes adds to the bandwidth requirement of transmitted data (or equivalently to storage), which is undesirable. Using robust error correcting codes also increases the processing overhead and complexity of implementation of the transmitter and receiver. Therefore in most applications—including video streaming applications or digital video broadcasting—the error control codes used do not permit all transmission errors to be corrected. Consequently, some transmission errors do occur. Unfortunately, in image and video transmission, some of these errors may sometimes result in noticeable artifacts that are displeasing to the eye. Obviously, noise on the transmission channel increases the likelihood of bit errors in the received video stream.
When errors are detected in received images and video, the receiver typically attempts to correct the errors, or at least reduce their undesirable effects. However, this often may not always lead to a subjectively acceptable outcome. For example, in color image or video transmission, color images are typically transmitted and received as pixels with color components (Y, Cb, Cr) in the YCbCr color-space representing the luma Y and chroma Cb, Cr. At the receiver, these components are converted to their equivalents in the RGB color-space which is typically used by digital displays.
For a receiver that uses 8-bit per color component in RGB space, each color component (R, G, B) ranges from 0 to 255. In the presence of transmission errors however, received YCbCr components may map to RGB components that are invalid—(i.e., with one or more color components are outside the permissible bounds). In this case, erroneous values are often truncated to the nearest acceptable value for the color component. Unfortunately however, this often leads to very noticeable artifacts. Very bright colors that standout in an otherwise demure image are very visible and distracting to a viewer and therefore undesirable.
Accordingly, an improved method of processing received digital color images is needed to reduce artifacts that result from transmission errors.